Field of the Invention
The present invention relates to a hollow stepping motor in which a hollow rotor is rotated by magnetic fields generated in a fixed barrel, and further relates to a lens device and an imaging device using this stepping motor.
Description of the Related Art
In recent years, an imaging device (electronic camera) utilizing a solid-state image sensor is incorporated in a small-sized terminal equipment of a cell-phone, a PDA and so forth. The solid-state image sensor is, for example, a CCD image sensor and a CMOS image sensor. The electronic camera converts a subject image, which is optically obtained through a taking lens, into an image signal by the solid-state image sensor to electronically capture and record the image. Shooting functions of the electronic camera have been improved as the solid-state image sensor is downsized and as pixel density thereof increases.
For instance, Japanese Patent Laid-Open Publication No. 59-109007 discloses a device in which a taking lens is moved to perform focus adjustment. This device comprises a movable barrel containing the taking lens, and a rotary barrel engaging with the movable barrel via a cam mechanism. By rotating the rotary barrel, the movable barrel is driven in an axial direction to move the taking lens. The device further comprises a fixed barrel surrounding the rotary barrel. The fixed barrel and the rotary barrel constitute a hollow stepping motor of a claw-pole type so that space efficiency is improved.
This kind of the hollow claw-pole-type stepping motor is constituted of a rotary barrel (rotor) 1 and a fixed barrel (stator) 6 such as shown in FIG. 18, for example. The rotary barrel 1 comprises permanent magnets of north pole and south pole, which are alternately arranged on its circumference. The fixed barrel 6 comprises first and second coil portions 2 and 4 respectively having a built-in coil.
As to the first coil portion 2, the coil is contained in a yoke made of a magnetic material of iron and so forth. The yoke has a gap formed in a rectangular-wave shape. In virtue of the gap, teeth 2a and 2b meshing with each other are formed at an inner surface of the first coil portion 2.
When a current flows in the first coil portion 2 in a forward direction (clockwise direction in the drawing), concentric lines of magnetic force are generated around the current (so-called right-handed screw rule). The generated line of magnetic force passes through the inside of the yoke made of the magnetic material, and is discharged into the air after reaching the tooth 2b. The discharged line of the magnetic force passes through the gap, and enters the yoke again from the tooth 2a. Thus, magnetic fields of south pole and north pole occur at the teeth 2a and 2b respectively. In contrast, when the current flows in the first coil portion 2 in a backward direction (counterclockwise direction in the drawing), the line of magnetic force is reversed. Thus, the magnetic fields of north pole and south pole occur at the teeth 2a and 2b respectively.
Similarly, teeth 4a and 4b are formed at an inner surface of the second coil portion 4. When the current flows in the second coil portion 4 in the forward direction, the teeth 4a is magnetized in south pole and the tooth 4b is magnetized in north pole. When the current flows in the backward direction, the tooth 4a is magnetized in north pole and the tooth 4b is magnetized in south pole. Incidentally, the first and second coil portions 2 and 4 are disposed in a state that the teeth 4a and 4b of the second coil portion 4 are positioned so as to be shifted relative to the tooth 2a and 2b of the first coil portion 2 by a half of the teeth.
For rotating the rotor 1 in the forward direction, it is performed first to let the current flow in the first coil portion 2 in the forward direction, such as shown in FIG. 19A. When the current flows in the first coil portion 2 in the forward direction, the teeth 2a is magnetized in south pole and the tooth 2b is magnetized in north pole to respectively attract the counterpart of the magnetic poles of the rotor 1. Successively, it is performed to let the current flow in the second coil portion 4 in the forward direction, such as shown in FIG. 19B. Since the teeth 4a and 4b of the second coil portion 4 are positioned so as to be shifted relative to the teeth 2a and 2b of the first coil portion 2 by the half of the tooth, the rotor 1 is attracted by each of the magnetically-polarized teeth 4a and 4b of the second coil portion 4 to rotate in the forward direction by an angle corresponding the half of the tooth.
Successively, it is performed in a similar way to let the current flow in the first coil portion 2 in the backward direction, such as shown in FIG. 19C. And then, it is performed to let the current flow in the second coil portion 4 in the backward direction, such as shown in FIG. 19D. After that, the operation shown in FIG. 19A is performed again. By repeating this sequence, the rotor 1 is rotated in the forward direction. In the meantime, for rotating the rotor 1 in the backward direction, it is performed first to let the current flow in the first coil portion 2 in the forward direction. Successively, it is performed to let the current flow in the order of the backward direction of the second coil portion 4, the backward direction of the first coil portion 2 and the forward direction of the second coil portion 4. By repeating this sequence, the rotor 1 is rotated in the backward direction.
However, the conventional hollow stepping motor of the claw-pole type uses two coil portions to rotate a single rotor. In case that a plurality of rotors are rotated, a number of the coil portions increases. Thus, there arises a problem in that a size of the device becomes larger.